Injection molded part quality can be affected by many factors.
For example, when molding material flows around an obstruction in a melt channel such as a valve pin, other obstruction, it sometimes cannot merge or knit back together very well on the other side of the obstruction. Many times the result is a so-called knit-line appearing on a molded part as a visible line or a weak area in the molded part susceptible to failure under stress. It is difficult to predict the severity of knit-lines in parts before they are molded.
Another problem occurs when changing between batches of differently colored molding material. Parts may have specks or streaks of the old material long after the new material has been introduced. Color changes can be addressed by designing flow channels to discourage material stagnation. However, in many cases, stagnation cannot be totally eliminated.
Improper decompression of molding material may also cause problems. In a decompression phase of an injection cycle, a plasticizing screw of an injection molding machine is retracted after the mold gate freezes. This action is also known as suck-back. Such plasticizing screw movement is sometimes necessary to allow residual holding pressure to be relived from the hot runner or other portion of the flow channel. Stringing or pulling of molding material from the mold gates can occur when the mold is opened to eject the newly molded articles if residual pressure in the flow channel is too high. The decompression phase of an injection cycle is time dependent and is sometimes ineffective when the flow channel is long or narrow. Increasing the decompression phase of the molding cycle may disadvantageously increase the total cycle time.